The present invention relates to fluidized bed reactors in general and, more particularly, to an apparatus and method for controlling the rate of transfer of heat from fluidized particles to heat exchange tubes immersed in the fluidized bed so as to control the overall rate of heat extraction from the bed and to maintain the temperature of the bed within a preselected range.
Fluidized bed reactors are well-known in the prior art. In a typical fluidized bed reactor, a bed of solid particles is housed in a chamber, the floor of which is formed of a perforated or slotted plate. A fluidizing gas is introduced into the bed upwardly through the floor of the chamber. The velocity of the fluidizing gas is maintained, by means of controlling gas pressure, above the minimum fluidization velocity, i.e., that velocity at which the entire bed of solid particles is suspended or floated above the floor of the chamber in the fluidizing gas.
If such a fluidized bed reactor as described above is utilized in a process wherein heat is generated within the bed, it is common practice to immerse within the bed one or more heat transfer tubes. A fluid is passed through the heat exchange tubes to absorb a portion of the heat generated in the bed thereby cooling the bed.
Because of the effective mixing of bed solids generated during the fluidization process, a fluidized bed will possess a thermal homogeneity which is useful in many processes. For example, one common application is the use of a fluidized bed reactor as a chemical reactor wherein the fluidizing medium reacts with the solid particles of the bed. As chemical reactions are often exothermic, a cooling fluid is circulated through the immersed heat exchange tubes to cool the bed. The rate of heat transfer to the immersed tubes is controlled to maintain the bed in a temperature range optimal for the continuation of the chemical reaction.
Fluidized bed reactors having heat exchange tubes immersed within the bed are also commonly used as ore roasters. As the roasting of ores, in particular sulfur containing ores, is generally an exothermic reaction, heat must be removed from the bed by circulating a cooling fluid through the immersed tubes. The rate of heat transfer to the immersed tubes is controlled to prevent the temperature of the ore bed from exceeding the fusion point of the ore or dropping below the minimum roasting temperature.
Because of their inherent potential as clean and efficient combustors of coal, fluidized bed reactors are increasingly being utilized as furnaces for steam generators wherein crushed or pulverized coal is the fuel. In a fluidized bed furnace, the coal to be burned is typically mixed with a sulfur sorbent and burned in a fluidized bed, the combustion air serving as the fluidizing gas. Water is circulated through immersed heat exchange tubes and evaporated therein to form steam. In such an application, it is desirable to control the rate of heat transfer to the immersed tubes in order to control bed temperature. A high bed temperature could result in fusion of the bed particles leading to defluidization, while a low bed temperature could cause the combustion process of the chemical reaction within the bed to cease.
The prior art contains a multiplicity of teachings of a variety of apparatuses and methods that have been designed to control the output of or the temperature of a fluidized bed. One such method, disclosed in U.S. Pat. No. 3,047,365, teaches using a direct heat removal means, such as water injection or excess air variation, in conjunction with immersed heat exchange tubes as a means for controlling bed temperature. The major portion of heat would be removed by the immersed tubes with water injection or excess air variation used to fine tune the heat removal. In accordance with the teachings of this patent, water is sprayed onto the surface of the bed or the amount of air used to fluidize the bed is increased in order to cool the bed whenever the bed temperature approaches the maximum permissible temperature. A disadvantage associated with using increased air flow to cool the bed is that the increased fluidizing flow and the increased pressure drop attendant with increased air flow to the bed causes increased sensible heat loss due to dilution and a drop in overall efficiency. The use of water injection exhibits the disadvantage of increasing the dew point of the gases leaving the fluidized bed which can lead to corrosion problems on exposed metal surfaces disposed downstream of the bed.
Another method of controlling the output of or the temperature of a fluidized bed taught in the prior art involves altering the bed height in order to reduce the amount of heat transfer surface immersed in the fluidized bed. U.S. Pat. No. 2,997,286 discloses a fluidized bed reactor wherein the vertical spacing between the immersed heat exchange tubes varies over the height of the reactor chamber, the tubes being closely spaced at the top of the chamber and widely spaced at the bottom thereof. The amount of heat exchange surface immersed in the bed is varied to control the rate of heat removal from the bed by decreasing or increasing the height of the bed. Although theoretically sound, this method has proven impractical in practice.
Another prior art scheme relies on altering the amount of heat transfer surface immersed in the fluidized bed as a means for controlling the rate of heat removal from the bed. In U.S. Pat. No. 3,387,590, the rate of heat removal is reduced by slumping, i.e., deactivating, portions of the fluidized bed. A major disadvantage of this scheme is that the design of the fluidized reactor is much more complex. The chamber must be modularized or an extensive ductwork and damper system must be provided to permit the fluidized air supply to be shut-off to various sections of the bed.
Another scheme known in the prior art is disclosed in U.S. Pat. No. 4,136,642. As taught therein, a second heat exchanger is disposed in the fluidizing air plenum and connected in fluid communication with the heat exchange tubes immersed in the fluidized bed. A portion of the steam generated in the immersed tubes is passed to the second heat exchanger to supply a source of heat to preheat the fluidizing air. If the temperature of the bed drops, the amount of steam being directed from the immersed tubes to the air plenum is increased thereby increasing the temperature of the fluidizing air so that the heat removed from the bed is recycled back to the bed. Alternatively, the amount of steam fed to the air plenum is decreased in the event that bed temperature increases .
Still another apparatus for regulating the rate of heat removal from a fluidized bed is disclosed in U.S. Pat. No. 4,177,765. Therein, the fluidized bed is equipped with a plurality of slidable sleeves circumscribing the heat exchange tubes disposed therein. The rate of heat removal from the bed is controlled by selectively extending or retracting the sleeves over the tubes thereby respectively shielding the tubes from or exposing the tubes to the fluidized bed. A major disadvantage associated with this apparatus is that a large antichamber must be provided adjacent to the fluidized bed to house the tube sleeves when they are retracted from the bed.
Therefore, in view of the need evidenced in the prior art, it is an object of the present invention to provide an apparatus and method for controlling the rate of transfer of heat from a fluidized bed to heat exchange tubes immersed therein having the capability to effectively and efficiently control the overall rate of heat extraction from a fluidized bed and maintain the temperature of the bed at a preselected level.
It is a further object of the present invention to provide such a method and apparatus for controlling the rate of heat transfer to the immersed heat exchange tubes in the form of an arcuate tube shield which partially shields the tube from exposure to the fluidized bed particles, the tube shield being rotatable about the tube to selectively shield and expose different areas of the tube surface.
A still further object of the present invention is to provide such a method and apparatus characterized in that the tube shield is rotated about the tube to selectively shield and expose different areas of the tube surface in response to the temperature of the bed.